Given the harsh nature and post-apocalyptic atmosphere of Tarraxian, I've started bringing in things from my Gamma World 2nd edition boxed set, purchased on Ebay earlier this year. While the concepts fit just fine, I find I'm having a bit of a problem with the conversion. I have followed the guidelines in the 1st edition DMG, but I find them somewhat lacking. Mainly, my problem with GW to AD&D game mechanics is the scale of hit points. A Gamma World character starts with 1d6 hit points for every point of Constitution they have. Assuming each hit die yields a mean of 3.5 hit points per die, that means that a Gamma World character with Con in the average spread (9-12) has around 31-42 hit points their first foray into the world. By contrast, D&D characters start with 1 (2 for rangers) and generally seem to have 3-6 hit points... (my characters have a bit higher since I start at 1st level characters at full hit points)
The monsters and (arguably) the weapons and hazards of GW are scaled for such characters. Badders, who have six hit dice and look to be the "starter" monsters for a beginning group of GW characters (who will have, on average, between 9 and 12 hit dice) are actually more like monsters for AD&D characters levels 4-6. Likewise, low intensity radiation that would hinder a Gamma World character will outright kill many of my AD&D characters.
Since I am in fact playing AD&D and not Gamma World, I want to keep things AD&D-centric in terms of damage and scale. After all, who knows when the party will decide to seek the way back to their own home world, right?
Edit: Tonight I pretty much ran the numbers as they were. This resulted in the druid gaining an unfortunate mutation (no pain sensation) and in the party being defeated by a security robot... luckily the bot was using knockout grenades and a paralysis rod. The characters actually had a brilliant plan involving an unseen servant and several live grenades, but unfortunately they didn't know their grenades were just knockout gas, to which the bot was obviously impervious. We'll pick up next time... who knows where our heroes will wake up?